Despite great progress of lithium-sulfur (Li-S) battery performance at the laboratory-level, both key parameters and challenges at cell scales to achieve practical high energy density require high-sulfur-loading cathodes and lean electrolytes. Herein, a novel carbon foam integrated by hollow carbon bubble nanoreactors with ultrahigh pore volume of 6.9 cm³·g^-1 is meticulously designed for ultrahigh sulfur content up to 96 wt.%. Tailoring polysulfide trapping and ion/electron transport kinetics during the charge-discharge process can be achieved by adjusting the wall thickness of hollow carbon bubbles. And a further in-depth understanding of electrochemical reaction mechanism for the cathode is impelled by the in-situ Raman spectroscopy. As a result, the as-prepared cathode delivers high specific capacitances of 1,269 and 695 mAh·g^-1 at 0.1 and 5 C, respectively. Furthermore, Li-S pouch cells with high areal sulfur loading of 6.9 mg·cm^-2 yield exceptional practical energy density of 382 Wh·kg^-1 under lean electrolyte of 3.5 μL·mg^-1, which demonstrates the great potential for realistic high-energy Li-S batteries.